1. Technical Field
This invention relates to regenerating the performance of fuel cells that use proton exchange membranes (PEM) as the electrolyte.
2. Background Information
It is well known that fuel cells of all types experience performance losses or decay during the course of their operation. Performance loss is a degradation in the voltage of the cell at a fixed current density or, conversely, a degradation of current density at a fixed voltage. Such performance losses may be the result of a variety of factors, including operating environment, component design, operating and maintenance procedures, and the kinds of materials used. Any means or method for reducing such losses must consider the impact on other important aspects of cell design, cell performance and cell operation. For applications such as powering automobiles, high performance levels and the means for attaining and retaining it must be viewed in conjunction with expectations for fuel cell life, low cost and ease of maintenance, all of which are critical for commercial success.
PEM (proton exchange membrane) cells are currently being developed by both large and small companies for automotive applications, and for that reason the identification, understanding and resolution of PEM cell performance decay has become very important. An example of a PEM fuel cell is described in commonly owned U.S. Pat. No. 6,024,848 to Dufner et al, incorporated herein by reference.
The present invention regenerates a PEM fuel cell, which degrades in performance during normal operation, by periodically operating the cell in a manner to cause the cathode potential to drop below normal operating levels. The invention is equally applicable to cells using pressurized and unpressurized reactants.
According to one embodiment of the present invention, PEM fuel cell performance losses caused by phenomena occurring during normal cell operation are recovered by periodically reducing the cathode potential to about 0.6 volts or less, and preferably to 0.1 volt or less. Once the cathode potential is reduced to the desired low level, it is maintained at or below that level for a short period of time. The lower the potential to which the cathode is brought, the more quickly regeneration will occur. After regeneration the cell, when returned to normal operation, will operate at a higher performance level. The regeneration is preferably done periodically to maintain high cell performance levels.
During testing of PEM cells that use platinum or platinum alloy catalyst, unacceptable performance losses were observed over time. The evidence suggests the losses are attributable to performance deterioration occurring during cell current production under normal operating conditions, which means conditions wherein the cathode potential is above about 0.66 volt. It was discovered that reducing the cathode potential to below 0.66 volt for a short period of time regenerates the cells to the point wherein they are again able to operate under normal cell operating conditions at significantly higher performance levels then prior to the regeneration procedure. Surprisingly, all or essentially all the performance losses that occur during periods of normal operation can be recovered by operating the cell briefly at a low cathode potential. Thus, the performance level of the cell may be maintained at a high level for an extended period of time.
In the specification and claims the phrase xe2x80x9cnormal cell operationxe2x80x9d means that the cell is operating at a cathode potential of at least 0.66 volt with a hydrogen containing fuel on the anode and an oxidant on the cathode to provide an electric current within an external electric circuit to power an electricity using device. (Note: Cathode potential equals cell voltage, plus anode potential, plus the product of cell current and PEM resistance. Thus, cathode potential is always slightly higher than cell voltage.)
The more frequent the periodic regeneration (e.g. the less the amount of time the cell operates under normal operating conditions between successive regenerations), the higher will be the average performance level of the cell over the course of normal operation. The preferred frequency of regeneration will depend upon the construction and operation of the cell, as well as its application. For some cells hourly or daily regeneration may be desirable. For others it may be best to regenerate the cell weekly or upon the occurrence of some event (such as during routine maintenance), or whenever performance of the cell under normal operating conditions falls below a pre-selected level. In an automotive application regeneration might automatically be performed each time the car is started, although such frequent regeneration is not likely to be required.
Although it is by no means certain, it is believed that the cell decay which this invention is intended to periodically reverse is the result of cathode platinum catalyst being converted to platinum oxides during normal cell operation. Platinum oxides do not have as high a catalytic activity for oxygen reduction as does platinum.
Consequently, there is a drop in cell performance over time. There is evidence that the conversion of platinum to platinum oxides occurs slowly, building to undesirable levels over the course of hundreds of hours of cell operation. It is believed that operating the cell at cathode potentials below about 0.6 volt in accordance with the teachings of the present invention results in the platinum oxides being reduced (i.e. the pure platinum returns), thereby improving the cell performance when normal cell operation resumes. The conversion of the platinum oxides back to platinum using the teachings of the present invention occurs at a much faster rate than the build up of platinum oxides.
One specific method for accomplishing the foregoing regeneration of the PEM fuel cell is to stop the flow of oxidant to the cell and disconnect the electric load. The cell is then connected to a power supply; and a hydrogen containing gas (preferably the same fuel as provided to the anode, such as essentially pure hydrogen) is flowed through the cell across both the anode and cathode. This forces any remaining oxidant from the cell and results in reducing the cathode potential. Once the desired low cathode potential of 0.6 volt or less (preferably 0.1 volt or less) is reached, the cathode potential is maintained at or below that low voltage for a sufficient period of time to cause the cell to revert to a condition wherein, when normal operation of the cell resumes, the cell has recovered a major portion of, and preferably all the performance it lost while operating normally. We refer to the forgoing method as the xe2x80x9chydrogen pumpingxe2x80x9d method since hydrogen ions are xe2x80x9cpumpedxe2x80x9d from the cathode to the anode through the PEM during the regeneration process.
Another method for practicing the present invention is to disconnect the electrical load from the stack; flow hydrogen on the anode; and flow an inert gas, such as nitrogen, on the cathode. Hydrogen will diffuse across the PEM to the cathode (due to the hydrogen concentration difference on opposite sides of the porous membrane) and cause the cathode potential to drop. Once the cathode potential falls to a preselected low value of no greater than 0.6 volt, and preferably no greater than 0.1 volt, the cell is maintained at or below the preselected value for a sufficient period of time to cause the cell to revert to a condition wherein, upon resuming normal cell operation, the cell operates at a performance level significantly higher than its performance level immediately prior to the regeneration procedure, and preferably to the performance level of the cell immediately subsequent to the most previous regeneration. In this manner the cell retains high level of performance over an extended period of time.
Yet another technique for reducing the cathode potential to accomplish the purposes of the present invention is as follows: disconnect the cell from its normal operating load; halt the flow of oxidant to the cathode; continue the flow of hydrogen containing fuel to the anode; and connect the cell to an auxiliary external resistive load. When the flow of oxidant to the cell is stopped, some residual oxidant will remain within or be accessible to the oxidant flow field within the cathode. This oxidant is quickly consumed by the electrochemical reaction at the cathode as the current flows through the auxiliary external resistive load, thereby causing a reduction in the cathode potential. As in the previously described embodiments, once a desired low cathode potential is reached, the potential is held at or below that level for a period of time sufficient to restore cell performance to earlier higher levels.
In another embodiment, after a period of normal cell operation the electric load is removed from the cell and both the anode and cathode are supplied with a flow of hydrogen containing fuel, such as essentially pure hydrogen. In this open circuit mode with hydrogen on both electrodes, the cathode potential will be quickly reduced to below 0.1 volt. As in the previously described embodiments, once the cathode potential reaches a preselected low level it is maintained at or below that level for a period of time sufficient to regenerate the cell. The cell is then reconnected to the load and may resume normal operation.
Another method for regenerating a cell in accordance with the present invention is to intentionally periodically operate the cell at a cathode potential at or below about 0.64 volt without taking the cell off-line (i.e. without disconnecting the cell from its primary load) and without operating the anode and cathode on anything but their usual fuel and oxidant. For that reason this method has certain advantages over previously described embodiments. Two techniques that may be used in this embodiment are to operate the cell for a short period of time at a high current density, or to briefly operate the cell at high oxidant utilization. Both of these techniques result in a lowering of the cathode potential, and both are more fully described below. The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.